Campylobacter jejuni is now recognised as one of the commonest causes of bacterial enteritis in man but the pathogenic mechanisms of the organism and the host response to infection are not yet fully understood. Clinical and pathological findings suggested that C. jejuni is an invasive organism and this was confirmed by the results of experiments with HeLa cell monolayers and chicken embryos. The chorioallantoic membrane of chicken embryos, aged 11-17 days of incubation, was inoculated with the organism and the oldest embryos proved to be the most resistant to infection, as indicated by the number of C. jejuni recovered from the heart and liver. A variety of C. jejuni strains, isolated from both, human and animal sources, was tested in chicken embryos but no significant differences between the various isolates were detected. In an attempt to develop a suitable laboratory animal model for C. jejuni infection the use of adult rats and mice was investigated. Following intragastric inoculation with C. jejuni the organism did establish in the gut. However despite the presence of substantial numbers of campy 1obacters in the gastrointestinal tract frank clinical disease did not develop in any of the animals. The mice tended to be more consistently colonised by C. jejuni and to remain Infected for a longer period than the rats. There were no marked differences in susceptibility to C. jejuni Infection between the two strains of mice used. In order to try to increase the susceptibility of the animals to C. jejuni gastric acidity and intestinal motility were modified experimentally. Such procedures did not give rise to increased recovery of the organisms from the gut or to the development of enteritis in the animals. Clearance experiments using mice and chickens demonstrated that very large numbers of intravenously injected C. jejuni could rapidly be cleared from the blood of these animals. The liver played a major role in the removal of the organisms from the bloodstream, although the liver and spleen cleared similar numbers per gram of tissue. There was no significant difference. between the clearance rates of the differentstrains of C. jejuni ; tested. Infant mice were more susceptible to colonisation by C. jejuni than adult animals but, like the adults, did not develop clinical disease. The infants were inoculated with several strains of C. jejuni, isolated from human and animal sources, and the findings confirmed the results of the chicken embryo and blood clearance experiments in that only minor differences between the strains were detected. The infant mice were also dosed with two strains of NARTC (C. laridis) but, unlike C. jejuni, they displayed only a limited ability to persist in the gut of the young animals. Infant mice were also used to investigate the possible protective effect of passively administered antibody on the ability of C. jejuni to establish in the gut. Previous infection of the dam with the organism did not appear to confer resistance to infection on the litter. Pre-opsonisation of the organisms with .specific immune serum also failed to significantly affect the course of the infection in infant mice.